M Track Audio Buffer Size

[7headed]

Using 512 as my buffer size keeps my tracks from distorting and making other annoying noises. But for some reason, every time I open my session to continue working, the buffer size keeps going back to 128. That's when I start getting noises. So I close Pro Tools, go the the M Audio settings and change it back to 512. I open Pro Tools and its back to 128 again. If I open the hardware settings while the session is open and try to change it there, the same thing happens. It won't stay at 512. Any ideas on how to fix this? Thank you.